The present invention relates to an extruder, and more particularly to an automatic filter screen exchange apparatus for an extruder.
An automatic filter screen exchange apparatus for an extruder has such a function as well-known in the art that impurities contained in plastic materials to be extruded from the extruder are filtered by the filter screen and when the meshes of the filter screen become clogged by an accumulation of impurities on the screen, the filter screen is then automatically exchanged by a fresh one without necessitating the stopping of the operation of the extruder.
Such an automatic filter screen exchange apparatus for an extruder is usually divided into two types, that is, one a rotary valve type and the other a slide type. However, both types have various defects: in the rotary valve type, such a constitution has been publicly known in which a changeover valve is provided to change flow passages so that filter screens are successively changed from one passage to other. However, such a constitution has generally complicated plastic resin flow passages, resulting in the stay of resin in the passages thereby causing the deterioration of the resin there. Further, since the changeover valve must be rotated under the state of being subjected to a high backpressure of resin within the extruder, the force required for the manipulation of the valve becomes too high.
Therefore, the changeover valve of a large dimension does not allow smooth operation, so that rotary valve type has a fundamental defect that it can be adopted only for extruders of a small limited capacity. On the other hand, in the slide type such a constitution has been widely used on account of its simplicity in construction in which a slide plate arranged at right angles to the axis of the extruder is provided with two sets of breaker plates and filter screens side by side, the slide being adapted so as to be shifted by a fluid cylinder in a direction orthogonal to the axis of the extruder to effect the exchange of the screen filters from one set to other. However, in such a constitution, since it causes a large surface pressure on shifting surfaces to form between the slide plate and a seal ring mounted to the extruder due to the high backpressure of the resin, the shifting of the slide plate during the operation of the extruder is made very difficult. Further, if the slide plate is shifted during the operation of the extruder, a leakage of a large amount of resin occurs momentarily so that a drop in pressure of the resin happens in the extruder, thereby an unevennes is generated in the resin extruded through a die mounted at the forward end of the extruder, resulting in extruded products having poor qualities. In addition, since in the slide type the breaker plate has a circular shape, when it is used for an extruder of a large capacity its thickness becomes naturally thick in terms of strength, so the pressure loss becomes correspondingly large.